Converter stall Speed

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Gotzy
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Converter stall Speed

Post by Gotzy » Mon Oct 16, 2017 10:38 am

Howdy Folk,

Just want a quick chat about converter stall speed. We got a little Frog Eye Sprite (approx 2,100lb) with a tunnel rammed iron headed 327 with a Big Mutha Thumper cam, powerglide with a 5,000 stall converter, 3.9:1 rear, 27.5" pie crusts.

We just had it on a rolling road and established that we weren't getting locked in wheel speed until about 5,600 - 5,700 rpm when the cam is nearly done and we've missed it's peak torque, 306hp to the rear wheels.

I believed at the time I bought the converter that due to it being a light vehicle I needed a higher stall converter hence the purchase but I suppose I was wrong. I know it's a very basic cam and converter mismatch now.

Comp Cam recommend a 2,800+ stall and I suppose my question is, if I got a 2,800 or so stall do you reckon I'd still get an extra 500 rpm flash out of it like my current converter meaning I really need a 2,300 stall for that cam? Or is my current converter misbehaving? It was only a cheapest Boss Hog converter.

One mate said slot in a standard converter since it's a light car but I don't agree, having said that I've just got myself confused with the weight thing I think

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Re: Converter stall Speed

Post by F-BIRD'88 » Mon Oct 16, 2017 9:29 pm

The weight of the car is irrelevant to what the converter flash stall speed at launch is going to be.
(Assuming you got traction.)
You got the right converter but the wrong cam. Get rid of the sissy (Hyd) cam.

Swap to a Isky Cams 201CC1 or 201547 or 201549 Be sure to use suitable correct valve springs.
Now the tunnel ram can work.

lefty o
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Re: Converter stall Speed

Post by lefty o » Mon Oct 16, 2017 10:02 pm

quite the mismatch in oyur set up, but yes your converter is wrong. vehicle weight does matter, talk to a good converter builder. box spec'd converters arent all that they should be.

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Re: Converter stall Speed

Post by tenxal » Tue Oct 17, 2017 1:54 pm

F-BIRD'88 wrote:
Mon Oct 16, 2017 9:29 pm
The weight of the car is irrelevant to what the converter flash stall speed at launch is going to be.
Take a 3,000 lb car and put 500 lbs in it. The 'flash' will be considerably higher as the convertor works harder to get the mass moving.

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Re: Converter stall Speed

Post by F-BIRD'88 » Tue Oct 17, 2017 3:18 pm

No it won't... The stall speed at launch will be the same.
Both are loaded the same at launch.

The flash stall speed is a function of engine torque .

More torque input = higher stall speed at launch.

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Re: Converter stall Speed

Post by F-BIRD'88 » Tue Oct 17, 2017 3:39 pm

You can add 2000 lbs weight . The flash stall at launch will be the same rpm.

The top end converter coupling % will be slightly different
if the gears are the same at both weights because the
top end trap rpm and car speed and top engine rpm will be different.

But the launch stall rpm will be the same .

Change the engine torque input and tge stall will change
+\-..

The math formula for this is the simplified " K Factor formula"
Google it.

If you want a lower stall speed then change the converter.
If it us just a hot rod not a race car then try a Off the shelf
10" "3500 stall" street strip converter.

The stall behind a 327 will be close to that.
If a 383 cid the stall will be a bit higher.
More engine torque loads the converter harder at launch
thus a higher stall speed.

On this converter if the coupling % at max rpm (say 6500 rpm) is 7 to 10 % it is correct.
The converter never does lock up 100%.

On this car change the cam and the car will be a lot faster
and peak engine torque will be right near your stall speed rpm too.
Any one of the cams I listed will be a lot better.

night and day

(if changing the converter use a 10" 3500 stall for that thumper cam not 2800.)

If you want to go fast change the cam.
Its the cam.

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Re: Converter stall Speed

Post by F-BIRD'88 » Tue Oct 17, 2017 4:15 pm

Further: If the goal is to go faster in the 1/4 mile then this car does not have enough gear ratio.

The gear ratio needs to be high enough so that at the top end of the track the engine rpm is about 2000 rpm HIGHER rpm than the stall speed seen at launch ( "flash stall speed)

That means you want o be trapping at about 7000rpm.

At 2100 lbs and 306 chassis dyno HP the car should go 118 to 122 MPH.
Correct gear will be about a 4.56 gear for this.
Now the car will go a lot quicker and the converter will function correctly
with about a 7 percent coupling % at high rpm thru the traps.

The stall speed will be the same but the car performance will be a lot better.
One of the Isky cams listed will give the engine the top end rpm capability and power to
trap at 7000rpm. The engine power curve will be a LOT Better with more HP
and the car will fly.

Put the new cam in and take the car to the track and run it.
What is the trap speed and engine tach rpm thru the traps?

The speed will be about 122 MPH +/- and the engine max rpm will be a bit too LOW.
(remember you want the max engine rpm at the top end about 2000 rpm HIGHER than your flash stall speed rpm)
Now change the gearing for this. The power glide needs correct gearing.
The converter will work like you never seen and the car will go better too.

It will then "drive" better on the street too.

if the dyno test is accurate and car weight is accurate I predict a 4.56 gear is right.
Small difference will still want the same gear.

Ditch the sissy rabbit hyd cam for one of the Isky solids and gear the car correctly
based on track MPH and engine trap rpm and it will FLY.
( gear it to trap at about 7000 rpm) 3.91's is wrong.

And it will "drive" great around town too.

(Take note: Even if you change the converter, the 3.91's will still not be right.
This car needs to trap at about 7000rpm. correct gears will give you this.
The power glide is very critical on getting the correct gearing.
When the gearing is correct for the car the converter performs as designed too.
So......)
Last edited by F-BIRD'88 on Tue Oct 17, 2017 4:31 pm, edited 1 time in total.

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Re: Converter stall Speed

Post by F-BIRD'88 » Tue Oct 17, 2017 4:26 pm

If you got tiny 390 450 or 500cfm carbs on the tunnel ram and want to go fast
2x750 carbs is none too much. Tiny carbs and hyd sissy cam will really limit the tunnel ram perf.
The tunnel ram wants to eat.

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Re: Converter stall Speed

Post by lefty o » Tue Oct 17, 2017 5:10 pm

like i said, talk to a good converter maker. there is a reason they ask for the info they do, including vehicle weight.

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Re: Converter stall Speed

Post by F-BIRD'88 » Tue Oct 17, 2017 7:39 pm

You are confusing the initial "flash stall speed" at launch with how the converter loads and operates at high road speed and high engine rpm and how it "couples" and where on the track it is going to "lock up"
This is the point where it stops being a Torque converter and becomes a "fluid coupling". and no longer is multiplying torque ("stall mode")
The stator in the converter is no longer stationary and spins at more or less engine rpm.
This point of down track "lock up" ( which is never 100% 1 to 1) is a function of car weight and gear ratio and wind resistance too.

But this is not "stall speed" and you will not get it right until the car's rear axle gear ratio is correct.
Right now the car is under geared. Many trans/ converter guys do not understand how this all works.
get the 1/4 mile trap rpm about 2000 rpm HIGHER than the observed launch stall speed is and it will be about right and the converter will work as designed and intended. and "lock up"

Flash stall speed is a function of engine torque input.

(Even if this is just a street car-hot rod) take the car to the track and tune it for best 1/4 mile track MPH.
With that 1/4 mile track MPH info and the ENGINE RPM seen at the finish line you can then calculate the
correct axle gear ratio to get the engine trap rpm correct .

You can watch the tachometer on initial drag launch and see the true "flash stall speed RPM on launch" ( Assuming tire traction)
Take this launch stall rpm and ADD 2000 rpm to it. EG: 5200 Real rpm stall +2000 rpm ='s 7200 RPM

(with the 391's and 27.5" tires) the trap rpm will be too low.
Now -re gear the car to 1/4 mile "trap" at or real close to 7200 rpm..

Thus the need for a solid lifter cam like the Isky solid cams listed.
The car, engine (with tunnel ram) and trans/5000 stall converter will be very happy and the car will GLH and drive way better too.
The "5000 stall" torque converter will suddenly "lock up" and "couple" and perform like intended.

This rough But accurate guideline rule of about a 2000 rpm spread between the real on track Launch flash stall speed and the engine rpm thru the traps (1/4 mile) is always Very close to correct.

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Re: Converter stall Speed

Post by lefty o » Tue Oct 17, 2017 8:50 pm

im not confusing anything!

tenxal
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Re: Converter stall Speed

Post by tenxal » Tue Oct 17, 2017 8:50 pm

F-BIRD'88 wrote:
Tue Oct 17, 2017 3:39 pm
You can add 2000 lbs weight . The flash stall at launch will be the same rpm.
Dude...... :roll:

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Re: Converter stall Speed

Post by lefty o » Tue Oct 17, 2017 9:08 pm

tenxal wrote:
Tue Oct 17, 2017 8:50 pm
F-BIRD'88 wrote:
Tue Oct 17, 2017 3:39 pm
You can add 2000 lbs weight . The flash stall at launch will be the same rpm.
Dude...... :roll:
:lol:

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Re: Converter stall Speed

Post by Geoff2 » Wed Oct 18, 2017 5:16 am

The vehicle weight has to affect stall speed & flash stall. Extreme example, but imagine if the car weight was 10,000 lb; more tq will be needed to move it from rest, & that is going to result in more c'ter slippage to get the needed tq....& higher stall speeds.

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Re: Converter stall Speed

Post by tenxal » Wed Oct 18, 2017 6:12 am

Gear ratio also influences convertor 'flash'.

If you take a set of 4.56's out and change to 5.38's (for example), the convertor 'flash' will be lower as the car moves easier with the 5.38's. Same with transmissions...a change from a 2.08 low gear to a 2.75 will result in a lower 'flash' rpm.

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